The reason why neither No.400 Squadron or No.268 Squadron RAF show up in Rawlings book, is that they are not fighter Squadrons, but Army Co-operation Squadrons. They appear therefore in "Coastal, Support and Special Squadrons of the RAF and their Aircraft" by Rawlings. Similarly, for the Franks book, until the Army Co-operation Command Squadrons were temporarily transferred to Fighter Command on1 June 1943, before they were transferred to 2TAF when it was formed, they did not fall within the parameters for his books on Fighter Command losses. After 1 June 1943 date, you will see losses for these Squadrons listed in the Franks books.

Think of them if you will as Reconnaissance squadrons flying armed fighter type aircraft, primarily at low level.

Quite a list of Squadrons flying the Allison engined Mustangs (and earlier Curtiss Tomahawks) that do not appear in the Franks books until after 1 June 1943.

in the boocklet "Strangers in a Strange Land" H.H. Stafer(?)Squadron Signal where photos from an "Beuteluftpark" near Paris showing photos from an Mustang I in relativ good condition. But I don't have the book! Maybe on of the users here?
h.